(a) Field of the Invention
This invention relates to shape-memory alloy (SMA) actuators and other actuators using electromechanically active materials [collectively referred to in this application as SMA actuators] and to methods for their control. In particular, this invention relates to SMA actuators that are capable of miniaturization to achieve fast (sub-second) response, and to control methods for SMA actuators in general, and also in particular for the miniaturizable SMA actuators of this invention for low power consumption, resistance/obstacle sensing, and positional control.
(b) Description of Related Art
A class of materials was discovered in the 1950s that exhibit what is known as the shape memory effect. See, for example, K. Otsuka, C. M. Wayman, xe2x80x9cShape Memory Materialsxe2x80x9d, Cambridge University Press, Cambridge, England, 1998, ISBN 0-521-44487X. These materials exhibit a thermoelastic martensite transformation; i.e. they are pliable below a certain transition temperature because the material is in its martensite phase and can be easily deformed. When their temperature is raised above the transition temperature the material reverts to its austenite phase and its previous shape, generating a large force as it does so. Example of such materials are approximately 50:50 atom percent titanium-nickel (TiNi) alloys, optionally containing small quantities of other metals to provide enhanced stability or to alter the martensite-austenite transition temperatures; and these can be formulated and treated to exhibit the shape memory effect. Other such alloys include Cu/Al/Ni and Cu/Al/Zn alloys, sometimes known as xcex2-brasses. Such alloys are generically referred to as shape memory alloys (SMA) and are commercially available from a number of sources in wire form, with diameters from as low as 37 xcexcm to 1 mm or greater. See, for example, Dynalloy Corp., xe2x80x9cTechnical Characteristics of Flexinol Actuator Wiresxe2x80x9d, Technical Information Pamphlet Dynalloy Corp., 18662 MacArthur Boulevard, Suite 103, Irvine Calif. 92715, USA.
SMA wires are wires of shape memory alloy that are treated such that they can be easily stretched along their longitudinal axis while in the martensite phase, thus re-arranging their atomic crystalline structure. Once stretched they remain that way until they are heated above their austenite transition temperature, at which point the crystalline structure is restored to its original (remembered) austenite configuration. This reversion not only returns the wire to its original length, but also generates a large force, typically on the order of 50 Kgf/mm2 cross-sectional area, depending on the alloy and its treatment. Because of the large available force per cross-sectional area, SMA wires are normally produced. in small diameters. For example, a 100 xcexcm diameter wire can deliver about 250 g of force. To obtain more force, thicker wires or multiple wires are required.
Although SMAs have been known since 1951, they has found limited commercial actuator applications due to some inherent limitations in the physical processes which create the shape memory properties. This lack of commercial applications is due to a combination of the following factors:
(1) Limited Displacement
A TiNi SMA wire can contract by at most 8% of its length during the thermoelastic martensite to austenite transition. However, it can only sustain a few cycles at this strain level before it fails. For a reasonable cycle life, the maximum strain is in the 3-5% range. As an example, for an actuator with reasonable cycle life, it requires over 25 cm of SMA wire to produce 1 cm of movement.
(2) Minimum Bend Radius
An obvious solution to packaging long lengths of SMA into small spaces is to use some kind of pulley system. Unfortunately SMA wires can be damaged if they are routed around sharp bends. Typically an SMA wire should not be bent around a radius less than fifty times the wire diameter. As an example, a 250 xcexcm diameter wire has a minimum bending radius of 1.25 cm. It should be noted that the term xe2x80x9cminimum bending radiusxe2x80x9d as used here means the minimum radius within which an SMA wire can be bent and still be capable of repeated austenite-martensite cycling without damage. The addition of a large number of small pulleys makes the system mechanically complex, eliminating one of the attractions of using SMA in the first place. Also the minimum bend radius requirement places a lower limit on actuator size.
(3) Cycle Time
An SMA wire is normally resistively heated by passing an electric current through it. The wire then has to cool below its transition temperature before it can be stretched back to its starting position. If this cooling is achieved by convection in still air, then it can take many seconds before the actuator can be used again. The 250 xcexcm wire discussed above has a best cycle time of about 5 seconds or more. Thus, as an example, Stiquito, an SMA powered walking insect [J. M. Conrad, J. W. Mills, xe2x80x9cStiquito: Advanced Experiments with a Simple and Inexpensive Robotxe2x80x9d, IEEE Computer Society Press, Los Alamitos Calif., USA, ISBN 0-8186-7408-3] achieves a walking speed of only 3-10 cm/min. Since the rate of cooling depends on the ratio of the surface area of the wire to its volume, changes in wire diameter dramatically affect the cycle time.
To overcome these limitations designers of SMA based actuators have typically used long straight wires or coils. See, for example, M. Hashimoto, M. Takeda, H. Sagawa, I. Chiba, K. Sato, xe2x80x9cApplication of Shape Memory Alloy to Robotic Actuatorsxe2x80x9d, J. Robotic Systems, 2(1), 3-25 (1985); K. Kuribayashi, xe2x80x9cA New Actuator of a Joint Mechanism using TiNi Alloy Wirexe2x80x9d, Int. J. Robotics, 4(4), 47-58 (1986); K. Ikuta, xe2x80x9cMicro/Miniature Shape Memory Alloy Actuatorxe2x80x9d, IEEE Robotics and Automation, 3, 2151-2161 (1990); and K. Ikuta, M. Tsukamoto, S. Hirose, xe2x80x9cShape Memory Alloy Servo Actuator with Electrical Resistance Feedback and Application for Active Endoscopexe2x80x9d, Proc. IEEE Int. Conf. on Robotics and Information, 427-430 (1988). Clearly, in many applications, especially where miniaturization is desired, it is impractical to use long straight wires. Coils, although greatly increasing the stroke delivered, significantly decrease the available force; and, to compensate for the drop in force, thicker wires are used which reduce the responsiveness of the resulting actuator, making it unsuitable for many applications.
Other mechanisms commonly used to mechanically amplify the available displacement, such as those disclosed in D. Grant, V. Hayward, xe2x80x9cVariable Control Structure of Shape Memory Alloy Actuatorsxe2x80x9d, IEEE Control Systems, 17(3), 80-88 (1997) and in U.S. Pat. No. 4,806,815, suffer from the same limitation on available force, again leading to the requirement for thicker wires and the attendant problems with cycle time.
As discussed above, SMA materials can be used as the motive force for an actuator [See, for example, T. Waram, xe2x80x9cActuator Design Using Shape Memory Alloysxe2x80x9d, 1993, ISBN 0-9699428-0-X], whose position can be controlled by monitoring the electrical resistance of the alloy. See, for example, K. Ikuta, M. Tsukamoto, S. Hirose, xe2x80x9cShape Memory Alloy Servo Actuator with Electrical Resistance Feedback and Application for Active Endoscopexe2x80x9d, discussed above.
A common method of heating SMA actuators to their transition temperature is pulse width modulation (PWM). In this scheme, a fixed voltage is applied for a percentage of a pre-set period. As the percentage on-time to off-time in a single period (referred to as the duty cycle) is changed, the aggregate amount of power delivered to the SMA can be controlled. This scheme is popular because of the ease with which it can be implemented in digital systems, where a single transistor is all that is required to drive an actuator, obviating the need for digital-to-analog conversion and the associated amplifiers.
In a simple example, a PWM generator supplies PWM pulses to the SMA element at a duty cycle and period specified by a digital controller. During the off period of the PWM pulse, a resistance measuring system measures the resistance of the SMA which is sampled and then held in a sample-and-hold system. This measurement is made in the off cycle because the PWM pulse can be quite short and the controller might not sample the SMA when the pulse is on. Finally, the analog signal in the sample-and-hold system is converted to digital form by an analog-to-digital (A-D) converter, from which it can then be read by the controller. This value is then used by an algorithm in the controller to vary the duty cycle of the PWM generator to achieved a desired position of the SMA element. In systems with more than one SMA element, all of the systems other than the controller need to be replicated for each SMA element, which leads to large, complex and expensive control systems.
Several schemes have been proposed to avoid this replication. The most common is to multiplex the A-D converter across a number of sample and hold circuits, thus only requiring one A-D converter. Another scheme, described in U.S. Pat. No. 5,763,979, uses electronic switches in a row and column configuration to isolate a single SMA element and applies a PWM pulse to each element in turn. This allows for the resistance measuring, sample and hold and A-D subsystems to be shared across all actuators, and also has the advantage of reducing the number of wires required to interconnect the devices. Unfortunately the scheme also doubles the number of high current switching devices since each actuator requires two such channels as opposed to only one in the conventional scheme. These switches are normally the physically largest element of such control systems because of their need to dissipate substantial heat due to their high current operation. So, although this scheme reduces the number of wires, it actually increases the size and complexity of the controller subsystem.
The transition from the martensite (low temperature) phase to the austenite (high temperature) phase in SMAs does not happen instantaneously at a specific temperature but rather progresses incrementally over a temperature range. FIG. 1 shows the relationship between displacement and temperature, indicating the austenite start As and austenite finish Af temperatures, as well as the martensite start and finish temperatures Ms and Mf respectively. In the temperature range indicated by xcex94T the alloy consists of a mixture of austenite and martensite. As can be seen, substantially no change in length occurs below As, and substantially no further change in length occurs above Af, as the SMA is heated. Similarly, on cooling substantially no change in length occurs above Ms, and substantially no further change in length occurs below Mf, however, there is typically substantial hysteresis in the length-temperature curve. As discussed in K. Ikuta, M. Tsukamoto, S. Hirose, xe2x80x9cShape Memory Alloy Servo Actuator with Electrical Resistance Feedback and Application for Active Endoscopexe2x80x9d, discussed above, and U.S. Pat. No. 4,977,886, there is a relationship between the electrical resistance of an SMA component and its temperature, as is shown in FIG. 2, which is shown for an SMA having an Mf above room temperature. As can be seen, within the shaded region between Rmin and Rmax the resistance can be used as an analog for the SMA temperature and hence it is possible to deduce the percentage transformation between the two phases based entirely on the resistance value with no direct measurement of temperature, since the resistance-temperature curve does not display significant hysteresis. However, due to the large position-temperature hysteresis illustrated in FIG. 1, knowledge of the temperature alone is not sufficient to deduce position.
However, if two actuators are arranged in an antagonistic fashion, a number of schemes can be used to compensate for the hysteresis. A common scheme described in Dynalloy Corp., xe2x80x9cTechnical Characteristics of Flexinol Actuator Wiresxe2x80x9d and U.S. Pat. No. 4,977,886 uses the normalized resistance from both actuators in combination to compensate for the hysteresis. All of these position control schemes rely upon an a priori knowledge of Rmax and Rmin (see FIG. 2). These values change over time as the alloy ages, and also with environmental factors, such that the system has to be recalibrated before each use for useful position control. Calibration is achieved either by the attachment of external sensors to compute Rmax and Rmin at known measured minimum and maximum displacements or, as in U.S. Pat. No. 4,977,886, by applying a current large enough and long enough such that the temperature will exceed Af and record the minimum and peak resistances encountered. The former calibration scheme is impractical for many systems where continuous, low cost operation is required. The latter scheme relies upon knowledge of the physical dimensions of the SMA element, and also its current environment and state (e.g. austenite or martensite) so that the magnitude and duration of the calibration pulse can be calculated.
The disclosures of all documents cited in this section and elsewhere in this application are incorporated by reference into this application.
It would be desirable to develop SMA actuators that are capable of providing substantially the full force of the SMA wires comprising them while achieving a greater stroke (contraction) than is achievable by an SMA wire of the length of the actuator (stroke multiplication without significant force reduction); SMA actuators that are miniaturizable and fast acting; and economical and efficient control and sensing mechanisms for SMA actuators (including conventional shape memory alloy actuators as well as the stroke-multiplying SMA actuators of this invention) for low power consumption, resistance/obstacle/load sensing, and accurate positional control.
This invention provides stroke-multiplying shape memory alloy actuators and other actuators using electromechanically active materials [collectively referred to in this application as SMA actuators] providing stroke multiplication without significant force reduction, that are readily miniaturizable and fast acting, and their design and use; economical and efficient control and sensing mechanisms for shape memory alloy actuators (including conventional shape memory alloy actuators as well as the stroke-multiplying SMA actuators of this invention) for low power consumption, resistance/obstacle/load sensing, and accurate positional control; and devices containing these actuators and control and sensing mechanisms.
In a first aspect, this invention provides a stroke-multiplying shape memory alloy actuator. In embodiments of this first aspect of the invention, the actuator comprises multiple rigid members and shape memory alloy wires.
In a second aspect, the invention provides a stroke-multiplying shape memory alloy actuator comprising a single shape memory alloy wire.
In a third aspect, this invention provides multiplexed control and sensing mechanisms for shape memory actuators.
In a fourth aspect, the invention provides control and sensing mechanisms for, and methods for controlling, shape memory alloy actuators using resistive feedback, in which the change in resistance of the actuator with time as the actuator is energized is used to generate the control information for the actuator. These control and sensing mechanisms and methods may be used for calibration of actuators, executing position control functions, measuring applied loads on actuators, and detecting collisions or mechanical obstructions encountered. by, or system failures in, actuators. In a preferred control mechanism, measurement of the discharge time of a capacitor connected parallel to the actuator is used to measure the resistance of the actuator.